rspec/rules/S6322/java/rule.adoc

Many modification methods in the collection interfaces are optional.
Some implementations do not implement those methods and throw a runtime exception (`UnsupportedOperationException`).
To fix this issue, make sure you call modification methods on a collection implementation that supports them.

== Why is this an issue?

The Java Collections framework defines interfaces such as `java.util.List` or `java.util.Map`.
Several implementation classes are provided for each of those interfaces to fill different needs: some of the implementations guarantee a few given performance characteristics, some others ensure a given behavior, for example immutability.

Among the methods defined by the interfaces of the Collections framework, some are declared as "optional": an implementation class may choose to throw an `UnsupportedOperationException` when one of those methods is called.
For example, `java.util.Collections.emptyList()` returns an implementation of `java.util.List` which is documented as "immutable".
Calling the `add` method on this object triggers an `UnsupportedOperationException`.

=== What is the potential impact?

include::../../../shared_content/layc/exception-impact.adoc[]

== How to fix it

When calling a method labeled as optional, you should make sure that the implementation class on which the call is made indeed supports this method.

=== Code examples

==== Noncompliant code example

[source,java,diff-id=1,diff-type=noncompliant]
----
List<String> list = Collections.emptyList(); // The list implementation returned here is unmodifiable.
if (someCondition) {
  list.add("hello"); // Noncompliant; throws an UnsupportedOperationException
}
return list;
----

==== Compliant solution

[source,java,diff-id=1,diff-type=compliant]
----
List<String> list = new ArrayList<>();
if (someCondition) {
  list.add("hello");
}
return list;
----

or

[source,java,diff-id=1,diff-type=compliant]
----
if (someCondition) {
  return Collections.singletonList("hello");
}
return Collections.emptyList();
----

== Resources

=== Documentation

* https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/doc-files/coll-overview.html[Collections Framework Overview] in the Java documentation
* https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/List.html[List] in the Java documentation
* https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/Set.html[Set] in the Java documentation
* https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/Map.html[Map] in the Java documentation